<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .a {
            width: 200px;
            height: 200px;
            background-color: #f00;
            /* 边框粗细   边框颜色   边框样式 */
            /* border-width: 3px;
            border-color: #000;
            border-style: solid; */
            /* 简写  不分先后顺序 */
            /* 边框也要占位置 */
            /* solid 实线 */
            /* dashed 虚线 */
            /* dotted 点线 */
            /* border: 10px solid #000; */
            /* border-top: 10px solid #000; */
            border: 3px solid #000;
        }
    </style>
</head>

<body>

    <div class="a"></div>

</body>

</html>